FIG. 2 shows a cross-sectional view of a prior art SBA laser disclosed in "Extended Abstracts of 17th Conference on Solid State Devices and Materials, 1985, pp.63 to 66". In FIG. 2, reference numeral 11 designates a p type GaAs substrate. A p type Al.sub.0.43 Ga.sub.0.57 As layer 12 is disposed on the substrate 11. An n type GaAs current confinement layer 13 is disposed on the layer 12. A p type Al.sub.0.43 Ga.sub.0.57 As lower cladding layer 14 is disposed on the current confinement layer 13 and on the portion of the layer 12 not covered by current confinement layer 13. An undoped Al.sub.0.07 Ga.sub.0.93 As active layer 15 is disposed on the lower cladding layer 14. An n type Al.sub.0.43 Ga.sub.0.57 As upper cladding layer 16 is disposed on the active layer 15. An n type GaAs contact layer 17 is disposed on the upper cladding layer 16. Reference numeral 18 designates a channel region, reference numeral 19 designates an active region, and reference numeral 20 designates a re-growth interface.
The production process of this laser device will be described.
First of all, a p type Al.sub.0.43 Ga.sub.0.57 As layer 12 is grown on the p type GaAs substrate 11 and an n type GaAs current confinement layer 13 is grown on the layer 12. A central portion of the current confinement layer 12 is removed by etching. The etching is stopped by the Al.sub.0.43 Ga.sub.0.57 As layer 13 because it has an etching speed that is much lower than that of the GaAs current confinement layer 13. Thereafter, a p type Al.sub.0.43 Ga.sub.0.57 As lower cladding layer 14, an undoped Al.sub.0.07 Ga.sub.0.93 As active layer 15, an n type Al.sub.0.43 Ga.sub.0.57 As upper cladding layer 16 and an n type GaAs contact layer 17 are successively grown. The lower cladding layer 14 is grown on the surface of the layer 12 that is not covered by the current blocking layer 13 and on the current confinement layer 13 by Metal Organic Chemical Vapor Deposition (MOCVD) method so that the stripe configuration is maintained. This MOCVD process is carried out at a temperature of 700.degree. to 800.degree. C. Thus, a planar region of the active layer 15, constituting the active region 19 is disposed opposite and spaced from the portion of the layer 12 that is not covered by current confinement layer 13. Although not shown in the figure, a p side and n side electrode are disposed on the substrate 11 and contacting layer 17 of the laser device, respectively.
The device will operate as follows.
In this laser device, when a voltage is applied between the p side and n side electrodes, a current is injected into the active region 19 through the channel region 18 at the aperture portion of the current confinement layer 13. Since the energy band gaps of the cladding layers 14 and 16 are larger than that of the active region 19, current is confined in the active region 19, and light having a wavelength corresponding to the energy band gap of the active region 19 is generated. On the other hand, since the refractive index of the active region 19 is larger than the refractive indices of the lower and upper cladding layers 14 and 16 which surround the active region 19, the generated light is confined in the active region 19. Accordingly, when the gain exceeds the loss at a low current, laser oscillation occurs.
In the prior art SBA laser of such construction, AlGaAs layers constituting the lower and upper cladding layers 14 and 16 and the active layer 15 are successively grown on the layer 12 after the aperture in the current confinement layer 13 is formed. Therefore, depending on the growth conditions, an oxide may remain on the surface of the layer 12 and excess arsenic should attach to the surface of the layer 12. Furthermore, the arsenic or zinc p type impurities in the layer 12 may diffuse out of the layer 12 because GaAs disassociates at a temperature of 650` C. depending on the ambient As pressure. Furthermore, the layer 12 may be irregularly thermally etched. Therefore, a potential barrier may be disadvantageously generated at the re-growth interface 20, increasing the operating voltage of the laser. The regrowth interface 20 may be non-planar itself so that the active region 19 is planar, thereby exciting higher order transverse modes of laser oscillation.